Ventricular arrhythmias are a major cause of sudden death, which accounts for approximately half of cardiac mortality. The His-Purkinje system is composed of specialized cells responsible for the synchronous activation of the ventricles. However, experimental studies show that the Purkinje system can be arrhythmogenic during electrolyte imbalance, after exposure to various drugs, and in myocardial ischaemia, during which Purkinje cells can survive in anaerobic conditions. Purkinje cells can generate both automatic and triggered focal rhythms, and their network configuration can accommodate re-entrant circuits. In humans, a variety of monomorphic ventricular tachycardias can be sustained within the architecture of the Purkinje branches. Furthermore, discrete Purkinje sources can serve as critical triggers of ventricular fibrillation in a wide spectrum of patients with structural heart disease or with an apparently normal heart. In drug-resistant cases of monomorphic and polymorphic Purkinje-related ventricular tachycardias, catheter ablation is a very effective treatment. The specific transcriptional signatures and functional properties of Purkinje cells, including their intracellular calcium dynamics, underlie their extreme arrhythmogenicity. However, the identification of vulnerable individuals remains challenging, and the molecular mechanisms of Purkinje-related arrhythmias have to be characterized further to enable the development of interventions to prevent lethal cardiac arrhythmias.

Abstract-We have investigated the subcellular spontaneous Ca 2ϩ events in canine Purkinje cells using laser scanning confocal microscopy. Three types of Ca 2ϩ transient were found: (1) nonpropagating Ca 2ϩ transients that originate directly under the sarcolemma and lead to (2) small Ca 2ϩ wavelets in a region limited to Ϸ6-m depth under the sarcolemma causing (3) large Ca 2ϩ waves that travel throughout the cell (CWWs). Immunocytochemical studies revealed 3 layers of Ca 2ϩ channels: (1) channels associated with type 1 IP 3 receptors (IP 3 R 1 ) and type 3 ryanodine receptors (RyR 3 ) are prominent directly under the sarcolemma; (2) type 2 ryanodine receptors (RyR 2 s) are present throughout the cell but virtually absent in a layer between 2 and 4 m below the sarcolemma (Sub-SL); (3) type 3 ryanodine receptors (RyR 3 ) is the dominant Ca 2ϩ release channel in the Sub-SL. Simulations of both nonpropagating and propagating transients show that the generators of Ca 2ϩ wavelets differ from those of the CWWs with the threshold of the former being less than that of the latter. Thus, Purkinje cells contain a functional and structural Ca 2ϩ system responsible for the mechanism that translates Ca 2ϩ release occurring directly under the sarcolemma into rapid Ca 2ϩ release in the Sub-SL, which then initiates large-amplitude long lasting Ca 2ϩ releases underlying CWWs. I n cells devoid of t tubules such as atrial and Purkinje cells (Pcells), excitation-contraction coupling (ECC) involves Ca2ϩ release from stores located near the sarcolemma and subsequent Ca 2ϩ -induced Ca 2ϩ release (CICR) along a lattice of sarcoplasmic reticulum (SR) enveloping the sarcomeres, which then activate myofibrils throughout the cell. [1][2][3][4][5] Drugs which affect SR function, such as thapsigargin and ryanodine, inhibit Ca 2ϩ activation of Pcells. 2,6 Conversely, spontaneous nonpropagating Ca 2ϩ release and Ca 2ϩ waves cause sarcolemma depolarization in both pacemaker cells and Pcells, which can lead to nondriven electrical activity even at normal [Ca 2ϩ ] o . 2,7-9 Abnormal Ca 2ϩ release in the network of Pcells may also be involved in lethal arrhythmias after myocardial infarction. 7,10 Previous observations suggested that micro Ca 2ϩ transients, spanning only a few micrometers and traveling over short distances, initiate cell-wide Ca 2ϩ waves (CWWs) which in turn induce nondriven electrical activity in a Pcell aggregate. 7 Here, we determined the mechanistic relationships between the different subcellular spontaneous Ca 2ϩ events in canine Pcells using confocal microscopy. Materials and MethodsEighteen aggregates of 2 to 6 cells were enzymatically dispersed from the Purkinje network of canine left ventricle (nϭ9) 7 and placed in a chamber on the stage of an inverted laser scanning confocal microscope (LSCM). Fluorescence was measured only in rod-shaped Pcells with typical junctional ends, clear striations, and membranes free of blebs. 2 Measurement and Analysis of Ca 2؉ TransientsConfocal line-scans were first positioned parallel t...